Light emitting diodes (LEDs) have typically been used in low power devices such as displays and annunciators. More recently, higher power LED chips have made possible LED illumination lamps which have begun to replace incandescent bulbs in applications such as automobile stop lights. In many low and high power applications the on-axis intensity of the LED must be maximized in order to maximize the perceived brightness of the LED.
Commonly available LED lamps, such as the Hewlett-Packard Company model HLMP-D101 lamp shown schematically in FIG. 1, use an axially centered radius dome lens to image the top surface of the LED chip onto a viewing plane. Since electrical power is provided through a wire bonded to the top surface of the chip, the lens also images the shadow cast by the bond pad. Unfortunately, this shadow falls directly on the longitudinal axis of the LED lamp and the on-axis light intensity is minimized as shown in FIG. 2A. A viewer looking at the LED lamp along the longitudinal axis would see a central dark spot surrounded by a ring of light as shown in FIG. 2B.
In accordance with an illustrated preferred embodiment of the present invention, a multiply conic lens is used to maximize the on-axis light intensity of the LED lamp by overcoming the effect of the bond pad shadow. In cross-section, the lens appears as two rotationally symmetric conic sections rather than as the single ellipse used in the prior art. For optimal illumination, ellipses may be used as the conic sections. In this case, the axes of the two ellipses are parallel to the longitudinal (or optical) axis and each ellipse has a focus located on the top surface of the LED chip. Unlike the centered single ellipse used in prior art lenses, the axes of the two ellipses are each translated away from the optical axis which intersects bond pad centered on the chip. The two ellipses of the lens create overlapping images of the chip top surface which combine to maximize the on-axis light intensity of the LED lamp.
In another illustrated preferred embodiment of the present invention, the center depression in the lens formed by the intersection of the two ellipses is flattened. This flattening greatly improves manufacturability of the lens and has only a small effect on the on-axis light intensity of the LED lamp.
In yet another illustrated preferred embodiment of the present invention, the axes of the two ellipses (which are each translated away from the optical axis) are tilted either inward or outward in order to modify or tailor the light intensity profile of the LED lamp.